If you're using RPC over HTTP in Outlook, first sign in to OWA and install the SSL certificate.

To install a certificate when visiting a self-signed OWA server on Vista, you need to perform the following steps:

Run IE as an administrator (Right-click the desktop icon)

Visit the site.

Click through the certificate error

Click the "Certificate Error" button in the address bar.

Click View Certificate

Click Install Certificate

Unlike on XP, you must click the “Place all certificates in the following store” radio button, and choose the “Trusted Root Certification Authorities” store. If you don’t do this, the certificate goes in your personal store, and it isn’t trusted by IE.